The described apparatus is an optical system for collecting and filtering light rays being received at wide cone angles and, more particularly, comprises a spherical lens, concentric lens and filter elements for accomplishing this function.
Typical laser optical systems in use assume that the received light will be singularly collimated; that is, will arrive at the lens by transversing a straight line from the point of origin. This assumption is not always applicable in laser communications or laser radar systems where scattered rays might require collection from wide cone angles. One example would be a laser communication link between an underwater observer such as a diver or the like and his support ship or aircraft.
In this case, because of wave action and because of the high hydrosol or suspended particulate concentrations in sea water, scattering occurs so that the light energy from a point source is received by the submerged observer at a plurality of angles. Similarly, ship to ship transmission through fog results in a scattering of the laser light at a plurality of angles. In both cases a wide angle receiver is required.
Further, the underwater observer may not know the location of the aircraft or support vessel, and would be required to open the field of view to take in light from an even wider angle. Of course, as the field of view is increased an increased amount of background light is also received and tends to mask the communication signal even more than at the optimum field of vision, F.O.V. In any event it is necessary then to filter the admitted or received light in a narrow spectral band about the signal band to discriminate the desired signal. Such filtering could be accomplished by the combination of a wide band semiconductor such as GaAs.sub.x P.sub.(1-x) in the near infra-red range and crystal mixtures of ZnS, ZnSe, and ZnTe in the visible to act as long wavelength bandpass filters in combination with a dye absorption band to act as a low pass filter, the combination producing a narrow band filter. The difficulty with these systems is that they each require a material development project to tailor a filter to each wavelength selected. That is, if a particular semiconductor and dye combination can be found at one wavelength its tuning range is limited and another different semiconductor dye combination is necessary for each laser line chosen. A more cost effective solution would be to use dielectric interference filters where proper tuning is accomplished by simply varying the deposition time. Dielectric filters although nominally priced are, however, spectrally inefficient unless the received light impinges on the filter surface at substantially right angles thereto. More specifically, spectral bandwidths of one nm with transmission efficiencies of fourty percent are routine. However, when part of the signal beam is not perpendicular to the interference surface, the apparent maximum transmission wavelength increases with increasing angle of incidence.
A solution is to design an optical system to be used either with ordinary light or lasers which would have a wide field of view and a narrow dielectric bandpass filter with some decrease of spatial resolution in the spherical image plane.